The recent growth of high-speed telecommunications has placed increasing demand for more signal bandwidth. While attempting to augment the available bandwidth of channels within a given transmission frequency range (e.g., 2.5 to five gigahertz range), the combined use of amplitude and phase modulation, such as may be found with orthogonal frequency division multiplexing (OFDM), has proven difficult to implement in practical applications.
More specifically, the delivery of improved spectral efficiency of transmitted signals in linear modulation schemes typically undergoes significant distortion of both phase and amplitude when the modulated signals are boosted by a power amplifier for transmission to a receiver. The distortion is especially prevalent in transmitters that employ power efficient, but nonlinear, radio frequency (RF) power amplifiers. As a result, linearization techniques have been developed to produce a desirable trade-off between a transmitter's efficiency and its linearity.
Cartesian feedback may be employed as a linearization technique for reducing distortion in a transmitter system. Cartesian feedback linearization compares in-phase and quadrature phase baseband input signals with distorted in-phase and quadrature phase baseband feedback signals that are usually demodulated from the RF power amplifier output. To provide a reliable comparison between the in-phase and quadrature phase input and feedback signals, their respective components must be substantially in phase. In many important cases, there is an unacceptably large phase difference between the in-phase and feedback signals due to feedback loop delay, which may include both RF delay (or propagation delay) and baseband delay. This typically results in crosstalk between the in-phase and quadrature phase feedback signals.
Propagation delay dominates the phase characteristics at RF and provides a maximum limit to the unit loop gain bandwidth. This propagation delay, which is introduced mainly by the power amplifier, usually varies with frequency and power level. To synchronize the baseband and feedback signals, a phase adjuster or a phase shifter is necessary to reduce the average phase shift. Adjustment of this phase shifter may be accomplished by a phase-alignment control block or phase controller.
It is not unusual for the phase controller to be required to accommodate a phase shift ranging from zero to 360° to compensate for phase adequately. This attribute makes implementation of the phase controller quite difficult, since overall performance of the transmitter is extraordinarily sensitive to the phase of the feedback signals.
Accordingly, what is needed in the art is a way to effectively control a phase adjustment associated with a transmitter employing Cartesian input and feedback signals that reduces feedback loop error and distortion associated with power amplification.